The damage of DNA bases by ultraviolet (UV) light is assumed to be responsible for the induction of mutations an the development of human skin cancers. To understand the mechanisms of UV damage and repair processes at the molecular level, we will apply the ligation-mediated polymerase chain reaction (LMPCR), a novel and uniquely sensitive technique for in vivo mapping of DNA adducts. We will determine the frequency of the two major types of UV-induced DNA lesions {cyclobutane dipyrimidines and (6-4) photoproducts} and their repair rates at each nucleotide position within several human genes. An increased adduct frequency and/ordecreased repair rates at specific nucleotide positions may create mutation hot spots. This hypothesis will be tested using the human HPRT gene. UV adduct frequencies and repair rates within the human ras proto-oncogenes will be compared with the frequency of mutations found in human skin cancers. Local chromatin structure and transcription may influence formation and repair of individual UV photoproducts. Meaningful adduction and repair maps absolutely require nucleotide resolution and also require knowledge of the local chromatin state for their interpretation. Adduct mapping at the DNA sequence level combined with high resolution chromosomal footprinting to localize DNA binding proteins should give a very detailed picture of the molecular mechanism involved in UV damage and repair in human cells.